My wife found this Watermelon Margarita recipe in “Glamour” and it’s pretty good:

4 cups chopped watermelon
Juice of 7 smaller limes
1/4-1/3 cup of sugar (to taste)
2 cups of ice cubes
1/2 cup of tequila
1/4 cup Grand Marnier or triple sec

I recommend adding the alcohol and ice to the bottom of the pitcher and the watermelon on top or it will never blend.